Fail-safe pulse repeater

ABSTRACT

The invention relates to a fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail. The signal repeater includes an input circuit electrically coupled to the electrically continuous rail to receive the signals to be repeated. A signal-repeating output circuit is electrically coupled to the rail to provide the repeated signal. Finally there is a control circuit which is electrically coupled respectively to the input circuit and the signal-repeating output circuit to thereby initiate the repeated signal while simultaneously providing a period of nonresponse for the repeater for a predetermined period of time after the signal to be repeated is received and repeated.

United States Patent Primary Examiner-Kathleen H. Claffy AssistantExaminer-William A. Helvestine [72] lnventor Richard D. CampbellHarmarville, Pa.

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AttrneysH. A. Williamson, A. G. Williamso Sotak 8 a w SDS 0 d N m L nPwm fla AFP 11:1 25 224 [.11

[73] Assignee Westinghouse Air Brake Company Swissvale, Pa.

ABSTRACT: The invention relates to a fail-safe signal repeater forreceiving and re [54] FAIL-SAFE PULSE REPEATER Claims, 1 Drawing Fig.[52] US.

peating a signal from and to an electrically continuous rail. The signalrepeater includes an input circuit electrically coupled to theelectrically continuous rail to receive the signals to be repeated. Asignal-repeating output circuit is electrically coupled to the rail toprovide the re- [51] lnt.Cl........

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178/70 R, T, 70 TS, 71

, 2 peated signal. Finally there is a control circuit which is electri-[56] References Cited UNITED STATES PATENTS 3,462,552 8/1969Dobermann.................. 3,499,985 3/1970 Rowlands vv v v v 11 ra l/7FAIL-SAFE PULSE REPEATER This invention relates to a signal repeater foruse in electrically continuous rail track circuits. More specificallythis invention relates to a fail-safe signal repeater for receiving andrepeating a signal from and to an electrically continuous rail. Thesignal repeater includes an input circuit electrically coupled to theelectrically continuous rail to receive the signals to be repeated. Asignal-repeating output circuit is electrically coupled to the rail toprovide the repeated signal. Finally there is a control circuit which iselectrically coupled respectively to the input circuit and thesignal-repeating output circuit to thereby initiate the repeated signalwhile simultaneously providing a period of nonresponse for the repeaterfor a predetermined period of time after the signal to be repeated isreceived and repeated.

in recent years the trend in track circuits and signaling systems thatemploy track circuits has been toward larger track circuits employingelectrically continuous rail. in many areas of the country there aremiles upon miles of rail where the traffic density is low and theemployment of elaborate signaling systems for these areas isprohibitively expensive, and as a consequence no signaling is provided.Furthermore, all presently known signaling systems require line wiresand/or breaks in one or both of the rails to establish signaling throughthe rails. The presence of signal attenuation in the rails placeslimitations on the length of the track circuits, thereby increasing thenumber of interruptions that must be made in the rails.

Therefore, if there could be an inexpensive uncomplicated signalingsystem that could employ the electrically continuous rails for stretchesof to miles or more without breaks in the rail and operate in afail-safe manner, such a system would answer the longfelt need of therailroading industry in these areas. Past signaling systems were doggedby the ever-present problem of signal attenuation which would require aninterruption in one or both of the rails in order that signals inducedat one end of the track circuit be repeated many times and eventuallytransmitted to the other end. To the longfelt need above noted thisinvention provides the answer by providing a unique signal repeater thatoperates in a fail-safe manner and requires no breaks in the rail. Therepeater of this invention now makes possible the attainment ofsignaling systems that may be of great length which do not require theuse of line wires or require breaks in the rail.

It is therefore an object of this invention to provide a failsafe signalrepeater that may be electrically coupled across a pair of electricallycontinuous conductors, such as track rails.

Another object of this invention is to provide a pulse signal repeaterthat will never fire spontaneously. Yet another object of this inventionis to provide a pulse signal repeater in which the internal gain mustnot increase so that weaker signals might trigger the repeater.

Still another object of this invention is to provide a pulse signalrepeater which has a period of nonresponse to received pulses whichoccurs after receiving a pulse and transmitting a repeated pulse.

Another object of this invention is to provide a pulse signal repeaterin which the period of nonresponse never gets shorter or becomes zero.

Another object of this invention is to provide a pulse signal repeaterwhich has an output pulse that never increases in height even when theoutput of the repeater is shorted.

in the attainment of this invention there is provided a signal repeaterwhich receives and repeats a signal from and to a pair of electricallycontinuous rails. The repeater includes an input circuit which iselectrically coupled to the rails to receive a signal desired to berepeated. The input circuit includes an amplifier which is transformercoupled to the rails to thereby receive and amplify the signal from therails. The amplifier has an output electrically coupled to a controlcircuit. The control circuit is electrically coupled respectively to theinput circuit and the signal repeating output circuit. Specifically, thecontrol circuit includes a triggered blocking oscillator electricallycoupled to the amplifier output of the input circuit to be triggeredthereby. The blocking oscillator has an output signal electricallycoupled to the signal-repeating output circuit. The blocking oscillatoroutput signal has a time length equal to the predetermined period oftime aforementioned. The blocking oscillator output signal provides theperiod of nonresponse for the signal repeater. The signal-repeatingoutput circuit includes therein a circuit which has a stored signaltherein as well as a switching circuit. The switching circuit iselectrically coupled respectively to the blocking oscillator output andthe circuit with the stored signal therein. The switch circuit isresponsive to the appearance of the blocking oscillator output signal toallow the stored signal to be delivered to the electrically continuousrail as the repeated pulse.

Other objects and advantages of the present invention will becomeapparent from the ensuing description of illustrative embodimentsthereof, in the course of which reference is had to the accompanyingdrawing in which the single FIGURE depicts a circuit diagram of thefail-safe pulse repeater of the subject invention.

A description of the above embodiment will follow and then the novelfeatures of the invention will be presented in the appended claims.

Reference is now made to the drawing which illustrates in block diagramform the basic component of a system which could typically embody theinvention to be described. in one of the most elementary systemsembodying the invention a transmitter coupled across the rails ll and 12would meet the basic need. The transmitter 23 is electrically coupledacross the rails via leads 38, 39. Across the rails are shown, but notreferenced with numerals, a plurality of capacitors between thetransmitter 23 and a unit 22, which is a repeater and the subject ofthis invention, which will repeat a signal delivered from thetransmitter 23 to the repeater 22. The repeater 22 will then amplify thesignal and retransmit a repeated pulse along the rails toward the rightwhere it will be repeated by an additional repeater which is coupledacross the rails and bears the designation R. At the right-hand end ofthe track circuit is a receiver 57 which receives the signal originallygenerated by the transmitter 23. The receiver 57 is coupled across therails by leads 36 and 37.

A complete detailed description of this system is set forth in mycopending application for Letters Patent of the United States, Ser. No.885,085, filed Dec. 15, 1969, for Long Length Track Circuit, andassigned to the assignee of the present invention. Accordingly, only abrief description of the system will be set forth hereafter. The systemdescribed will establish the environment in which the repeater to bedescribed more fully will operate.

Without going into the details of the transmitter 23, suffice it to saythat a pulse of the type shown above the rail 11, that is pulse 73, willbe generated by the transmitter and will be impressed on the rails 11and 12 over the leads 38 and 39 from the transmitter 23. These pulses ofthe type 73 will pass down along the rails and, as can be seen from theshowing immediately above the rail II, the pulse 73 becomes attenuatedas it passes along the rail and this factor of attenuation isillustrated by the pulses 74 and 75 which are shown to be decreasing insize due to the attenuation brought about by impedance present in therails.

Coupled across the rails 11 and 12 are a plurality of capaci-v torsreferred to hereinbefore, These capacitors improve the transmission ofthe pulses along the rails and their position and number are subject ofmy above-referred to copending application. Specific details of thepositioning of these unreferenced capacitors will not be made here butreference is made to this copending application for these details.

There is coupled across the rails a repeater 22 connected to the railsll and 12 by leads 13 and 14. it will be seen as one passes from theleft-hand side decreasing this drawing toward the right that, inaddition to repeater 22, there are a plurality of repeaters designatedby the general term R electrically coupled across the rails and eachpositioned such that it will receive an attenuated pulse, such as thepulse 75. Repeater 22,

for example, will in turn amplify and retransmit the repeated pulse 76to the rails 11 and 12. The repeated pulse 76 will in turn experienceattenuation as is graphically illlustrated by the pulses 77 and 78 whichillustrate a decreasing amplitude of the pulse due to attenuation.

For the practice of this invention the selection of narrow pulses hasbeen made in order that these narrow pulses permit the filtering out oflow frequency noise which is believed to predominate in track circuitsin general. In determining the positioning of the repeaters, as well asthe function that they must perform, it should be recognized that thepropagation time of pulses along loaded track is approximately 0.l4millisecond per thousand feet with a maximum attenuation at 10,000 feetfor an 0.8 millisecond pulse of about 22 db. and a minimum attenuation,for example in dry weather, of 3.7 db. Accordingly, if repeaters areconnected every 10,000 feet in wet weather, the output of each repeaterwill be attenuated by 22 db. before it reaches the input of the nextrepeater. In this case, if one is referring to repeater 22, then thenext repeater to receive a repeated pulse would be the next repeater Rpositioned to the right of repeater 22. If one were to allow a margin of3 db. for each repeater, then the repeater should be able to betriggered by an input signal 25 db. below its own output signal. in thesystem being described here, each repeater must be absolutely dead, thatis, unable to react, when the input pulse for some interval after it hasfired. This is because once the repeater, for example repeater 22, hasrepeated 2. pulse for some interval after it has fired. This is becauseonce the repeater, for example, repeater 22, has repeated a pulse anddelivered it into the rails 11 and 12, this repeated pulse will go inboth directions along the rails 11 and 12, and when the next adjacentrepeater R repeats a received pulse, this repeated pulse will pass bothto the right and left along the electrically continuous rails 11 and 12.In the event that the repeater 22 is not dead, that is, unable torespond to a repeated signal from repeater R just noted, then therepeater 22 would produce a signal and the obvious confusion of signalsthat would appear throughout the length of the track circuit becomes avery large problem. Accordingly, one must calculate the interval theserepeaters must be dead. ln calculating this interval of dead time onemust look at the problem in dry weather conditions where typically theattenuation is 3.7 db. for 10,000 feet of rail. Therefore, in 80,000feet, approximately 15 miles, a signal would suffer a minimumattenuation of 30 db. This would be db. below the specified input notedabove with reference to the sensitivity needed at each receiver. And,therefore, a repeater cannot directly trigger another repeater 80,000feet away but it can trigger intervening repeaters during dry weather,as has been noted earlier.

if a pulse is transmitted at time T=0 and this pulse comes from thetransmitter 23, the first repeater 22, which is for example 10,000 feetaway, will receive an attenuated pulse at T=l.4 milliseconds, and thenthis repeater 22 will fire. In a similar fashion, at T=2.8 millisecondsthe first repeater pulse will arrive back at the transmitter 23 andsimultaneously trigger the second repeater R referred to above. Thisprocess goes on and on so that a new pulse appears at 2.8 millisecondintervals. The first repeater pulse will travel 80,000 feet down thetrack in 11.2 milliseconds, triggering the ninth repeater. This ninthrepeater pulse will travel 80,000 feet back to the repeater 22 in 1 1.2millisecond.

By the computations of the preceding paragraph this signal will alwaysbe too weak to retrigger the first repeater. Each repeater musttherefore be dead for approximately 22.4 milliseconds after it is fired.In practice there is allowed a substantial margin and the time is set at30 milliseconds.

This repeater system is especially unique in that it is easy totroubleshoot and maintain. in dry weather one would merely connect atthe transmitter end of the track circuit an oscilloscope. One would thensee the transmitter pulse, which in this instance would be approximatelyvolts, followed by the first repeater pulse, which would be about 6.7volts, followed by the second, etc. This is a string or collection ofpulses each one two-thirds the size of its predecessor. if one repeaterhas failed or has a dead battery, then this pulse of the collection willbe missing. One thus can do the maintaining of the track circuit systememploying the repeaters embodying the invention during dry weather basedon oscilloscope measurements at the ends of the track circuit. It isimportant to note that the repeaters, which form the subject of thisinvention and which will be described in detail hereafter, do not loaddown the rails should one of them fail, for if any one of the repeatersfails, it neither subtracts from nor adds to energy on the rails.Accordingly, if the ballast resistance is not too low, this signal willleapfrog a dead repeater and trigger the next repeater down the line.

The repeater must meet certain basic requirements consistent with therequirements that are demanded by the Association of American Railroads.Accordingly, the repeater must be fail-safe in addition to providing theabove-noted 30 millisecond dead time. And the fail-safeness requiresthat the repeater must not fire spontaneously or, in the other hand, theinternal gain of the repeater must not increase so that weaker signalsmight trigger the repeater. Also, the dead time must never get shorteror decrease to zero. in concluding, the output pulse of the repeatermust never increase in amplitude.

With these basic understandings of the positioning of the repeatersalong the track, a study may now be made of the details of the repeaterwhich embodies the invention. The most elementary system employing therepeater of this invention would involve a single transmitter andreceiver, which would allow a track circuit of a very great length to bepresent. ln this environment the transmitter would place a pulse intothe track which would be repeated down the length of the track circuitto a receiver many miles away. Accordingly, should a train enter thelength of track thereby shunting the rails, the pulses would not reachthe receiver and there would be an indication of track occupancy.

Reference is now made to the drawing wherein there is illustrated acircuit diagram of a pulse repeater of the type referred to above. Therepeater to be described will have a dead time or time during which nopulse may trigger the repeater. The duration of the dead time and how itis accomplished will now be set forth. As can be seen in the drawing,the repeater 22, shown in dotted outline, is electrically connectedacross the rails 11 and 12 by leads l3 and 14. A negative goingattenuated pulse 75, described earlier, appears on lead 13 from rail 11and enters the pulse repeater 22. The negative going pulse 75 travelsalong lead 131; through a current limiting resistor 16. The pulse 75continues along lead 17 until it enters a stepup transformer 18, shownin dotted outline, when it undergoes a inversion and appears on baselead 19 as a positive going pulse, as is shown immediately above lead19. Connected to base lead 19 is a diode 25 electrically connected bylead 24 to lead 19 and to ground by lead 26. This diode providesprotection to the transistor Q1 by affording an electrical path toground in the event there are excessive pulse of reverse polarityvoltage or voltage surges due to external sources which may induce inthe rails 11 and 12 undesirable transients.

The transistor O1 is connected to a positive battery voltage source 48and to ground. The collector 27 of the transistor 01 is electricallyconnected to positive battery source 48 via lead 30, resistor 31, lead32, primary winding 33 of transformer 35, and leads 34, 47. The emitter28 of transistor O1 is connected to ground by lead 29. Before transistorQ1 conducts, the voltage at the collector 27 is at a positive level. Theappearance of the positive pulse on base lead 19 causes the transistor01 to conduct and the output across Q1 will follow the pulse on the baselead 19, thereby inducing in the primary winding 33 of the transformer35 a negative-going pulse which will in turn induce in winding 40 of thetransformer 35 a negative-going pulse which will appear on lead 41.Accordingly, signals are standardized by transistor 01, which delivers acurrent pulse of constant size for all inputs above threshold. Thetransistor Q2 has its emitter 44 electrically connected to the positivebattery source 48 via lead 46. The collector 45 of transistor Q2 iselectrically connected to ground via lead 49, transformer winding 50,leads 51, 52, and resistor 53. As was noted earlier there is anegative-going pulse present on lead 41,

which pulse will pass through the diode 42 to base lead 43 of 5 thetransmitter Q2. This negative-going pulse will trigger the transistor 02into conduction. As soon as transistor 02 begins to conduct, anincreased voltage will appear in transformer winding 50 which will,through regenerative feedback, cause the negative-going condition onlead 41 and base lead 43 to continue in a negative direction, therebykeeping the transistor Q2 conducting.

The transistor 02 and transformer 35 form a blocking oscillator whichwhen triggered with transistor Q2 conducting will remain conducting forthe time required to saturate the transformer 35. The selection of thetransformer of course will be determined by the amount of dead timedesired. in the instant application of the invention this would be 30milliseconds. During the conducting period of the blocking oscillator itcannot be retriggered, thus providing the dead time. It should be notedthat when transistor 02 turns off, the voltage on the lead 41 rises wellabove the potential of the positive battery supply 48. The series diode42 thereby protects the base emitter junction of transistor 02. Theseries diode 42 is preferred to a shunt diode approach because if ashunting diode were employed, while dead time might be increased by asmuch as l0 milliseconds due to discharge of the transformer 35 throughthe shunting diode, should the shunting diode fall off or becomedisconnected then the dead time would suddenly decrease by milliseconds.This would result in an unsafe condition especially if the 10millisecond loss cut into the desired dead time required under dryballast conditions.

The output from the blocking oscillator which is comprised oftransformer 35 and transistor 02 is a positive-going square waveappearing on lead 51. Electrically coupled to lead 51 is adifferentiator which is comprised on the lead 54, capacitor 55, gatelead 56, and resistor 60 which is connected to ground. The leading edgeof the square wave above noted is differentiated which results in apositive-going spike which will be employed to trigger the siliconcontrolled rectifier 59. A diode 61 is electrically connected to gatelead 56 and provides a similar protective function for siliconcontrolled rectifier 59 as was described with reference to diode andtransistor Q1. The diode 61 also assures an electrical path to groundwhen the trailing edge of the square wave pulse from the blockingoscillator is differentiated to produce a negative going spike. Thesilicon-controlled rectifier 59 is electrically connected in a circuitbetween the positive battery terminal 48 and ground via lead 64,resistor 65, leads 66, 67, the silicon controlled recrifier 59, andfinally to ground. When the spike pulse due to the differentiation ofthe leading edge of the square wave output from transistor Q2 of theblocking oscillator appears on gate lead 56, this triggerssilicon-controlled rectifier 59 into conducting. The silicon-controlledrectifier 59 once conducting will remain conducting until the voltagebias between the anode and cathode of the silicon-controlled rectifierfalls below zero, at which time the silicon-controlled rectifier 59 willcease to conduct.

At the instant the silicon-controlled rectifier 59 starts to conductthere will be present on capacitor 68, of a stored signal circuit, acharge due to the battery potential on terminal 48 electricallyconnected thereto by lead 64, resistor 65, and lead 66. The capacitorwill dump its charge through the silicon-controlled rectifier 59 toground and in so doing, because of the electrical connection of thecapacitor 68 by lead 69, inductor 70, and lead 71 to transformer 72,induce in the secondary winding of the transformer 72 an output pulsewhich approximates a half sine wave which will be carried by lead 13aand thence over lead 13 to rail 11 where it will appear as repeaterpulse 76.

lt should be kept in mind that the combination of the capacitor 68, theinductor 70, and the transformer 72 plays three important simultaneousfunctions. The first of these functions is the assurance that thesilicon-controlled rectifier 59 will experience a reverse polarity, thusinsuring that the silicon-controlled rectifier 59 will be turned off. Inaddition, there is established the size of the output current so thatthe circuit behavior is changed very little by a sort circuit across theoutput. This is accomplished because the current through the inductor 70and capacitor 68 is oscillatory, so that the voltage across capacitor 68will run down through zero during the pulse and then overshoot by asmall amount, c.g., a few volts. While this arrangement of an inductorand capacitor in an oscillating circuit may be considered to beinefficient from the standpoint that a portion of the available pulsestored by the capacitor is used up internally in the capacitor andinductor, the highly desirable feature of being able to short the outputwithout damage to the circuit is present. This is because the current isalmost wholly determined by the inductor 70. ln the railway environmentin which this invention is to be employed, the problem of a possibleshort between the turns of the inductor, which would result in a verylarge output pulse which is unsafe, is avoided because the inductor 70is made from a few turns of heavy, well-insulated wire wound on a coreand a nonshorting can.

While the invention has been shown and described with reference to apreferred embodiment thereof, it will be understood by those skilled inthe art that other modifications may be made therein without departingfrom the spirit and scope of the invention.

Having thus described my invention, what I claim is:

1. A fail-safe signal repeater for receiving and repeating a signal fromand to an electrically continuous rail, said signal repeater including,

a. input means electrically coupled to said electrically continuous railto receive said signal to be repeated,

b. Signal repeating output means electrically coupled to saidelectrically continuous rail to provide said repeated signal,

c. control means electrically coupled respectively to said input meansand said signal-repeating output means to thereby initiate said repeatedsignal while simultaneously providing a period of nonresponse for saidrepeater for a predetermined period of time after said signal isreceived and repeated.

2. The fail-safe signal repeater of claim 1 wherein said input meansincludes an amplifier, transformer coupled to said rail to receive andamplify said signal form said rail, said amplifier having an outputelectrically coupled to said control means.

3. The fail-safe signal repeater of claim 2 wherein said con trol meansincludes a triggered block oscillator electrically coupled to saidamplifier output to be triggered thereby,

said triggered blocking oscillator having an output signal electricallycoupled to said signal-repeating output means of a length equal to saidpredetermined period of time, said blocking oscillator output signalproviding said period of nonresponse for said signal repeater.

4. The fail-safe signal repeater of claim 3 wherein saidsignal-repeating output means includes a stored signal energy means aswell as a switch circuit, said switch circuit electrically coupledrespectively to said block oscillator output and said stored signalenergy means, said switch circuit responsive to the appearance of saidblocking oscillator output signal to allow said stored signal energy tobe delivered to said electrically continuous rail as said repeaterpulse,

5. The fail-safe signal repeater of claim 4 wherein said switch circuitincludes a differentiator electrically coupled to said blockingoscillator output as well as a gate, said gate electrically coupled tosaid stored signal energy means and controlled by the initialdifierentiation of said blocking oscillator output signal to therebyallow the release of said stored signal energy to said electricallycontinuous rail.

6. The fail-safe signal repeater of claim 5 wherein said gate is asilicon-controlled rectifier with its gate electrically coupled to saiddifferentiator and its anode electrically connected to said storedsignal energy means.

7. The fail-safe signal repeater of claim 6 wherein said stored signalenergy means is a series-connected capacitor and inductor with saidcapacitor electrically coupled to said anode of said silicon-controlledrectifier and with said inductor electrically coupled to said rails tothereby permit the shorting of the repeater without the risk of inducinga very large pulse to said rails and accordingly insure fail-safeoperation.

8. The fail-safe signal repeater of claim 7 wherein said signal to berepeated is a pulse.

9. The fail-safe signal repeater of claim 3 wherein said triggeredblocking oscillator is transformer coupled to said amplifier output,

said transformer couple including a primary, a saturable core, a firstsecondary, and a second secondary,

said triggered block oscillator further including a transistor with itsbase electrically coupled through a series diode to said first secondaryand the collector of said transistor electrically coupled to said secondsecondary,

said period of nonresponse occurring during a period of conduction ofsaid transistor when said amplifier output initially appears and saidsaturable core is saturating,

said series diode providing protection against excessive voltage surgescaused by the collapse of the field surrounding said saturable core atthe end of said period in which said saturable core is saturating, saidseries diode thereby insuring that said period of nonresponse is alwaysof the same time duration.

10, A fail-safe signal repeater for receiving and repeating a signalfrom and to an electrically continuous rail, said signal repeaterincluding,

a. input means electrically coupled to said electrically continuous railto receive said signal to be repeated, said input means having anamplifier, transformer coupled to said rail to receive and amplify saidsignal from said rail, said amplifier having an output electricallycoupled to a control means, said control means electrically coupled tosaid input means and a signal-repeating output means, said control meanshaving a triggered blocking oscillator electrically coupled to saidamplifier output to be triggered thereby, said triggered blockingoscillator having an output signal electrically coupled to said signalrepeating output means, said output signal having a length equal to apredetermined period of time, said blocking oscillator output signalproviding a period of nonresponse for said signal repeater, c. saidsignal repeating output means having a stored signal energy means aswell as a switch circuit, said switch circuit electrically coupledrespectively to said blocking oscillator output and said stored signalenergy means, said switch circuit responsive to the appearance of saidblocking oscillator output signal to allow said stored signal energy tobe delivered to said electrically continuous rail as said repeatedpulse.

1. A fail-safe signal repeater for receiving and repeating a signal fromand to an electrically continuous rail, said signal repeater including,a. input means electrically coupled to said electrically continuous railto receive said signal to be repeated, b. Signal repeating output meanselectrically coupled to said electrically continuous rail to providesaid repeated signal, c. control means electrically coupled respectivelyto said input means and said signal-repeating output means to therebyinitiate said repeated signal while simultaneously providing a period ofnonresponse for said repeater for a predetermined period of time aftersaid signal is received and repeated.
 2. The fail-safe signal repeaterof claim 1 wherein said input means includes an amplifier, transformercoupled to said rail to receive and amplify said signal from said rail,said amplifier having an output electrically coupled to said controlmeans.
 3. The fail-safe signal repeater of claim 2 wherein said controlmeans includes a triggered block oscillator electrically coupled to saidamplifier output to be triggered thereby, said triggered blockingoscillator having an output signal electrically coupled to saidsignal-repeating output means of a length equal to said predeterminedperiod of time, said blocking oscillator output signal providing saidperiod of nonresponse for said signal repeater.
 4. The fail-safe signalrepeater of claim 3 wherein said signal-repeating output means includesa stored signal energy means as well as a switch circuit, said switchcircuit electrically coupled respectively to said block oscillatoroutput and said stored signal energy means, said switch circuitresponsive to the appearance of said blocking oscillator output signalto allow said stored signal energy to be delivered to said electricallycontinuous rail as said repeater pulse.
 5. The fail-safe signal repeaterof claim 4 wherein said switch circuit includes a differentiatorelectrically coupled to said blocking oscillator output as well as agate, said gate electrically coupled to said stored signal energy meansand controlled by the initial differentiation of said blockingoscillator output signal to thereby allow the release of said storedsignal energy to said electrically continuous rail.
 6. The fail-safesignal repeater of claim 5 wherein said gate is a silicon-controlledrectifier with its gate electrically coupled to said differentiator andits anode electrically connected to said stored signal energy means. 7.The fail-safe signal repeater of claim 6 wherein said stored signalenergy means is a series-connected capacitor and inductor with saidcapacitor electrically coupled to said anode of said silicon-controlledrectifier and with said inductor electrically coupled to said rails tothereby permit the shorting of the repeater without the risk of inducinga very large pulse to said rails and accordingly insure fail-safeoperation.
 8. The fail-safe signal repeatEr of claim 7 wherein saidsignal to be repeated is a pulse.
 9. The fail-safe signal repeater ofclaim 3 wherein said triggered blocking oscillator is transformercoupled to said amplifier output, said transformer couple including aprimary, a saturable core, a first secondary, and a second secondary,said triggered block oscillator further including a transistor with itsbase electrically coupled through a series diode to said first secondaryand the collector of said transistor electrically coupled to said secondsecondary, said period of nonresponse occurring during a period ofconduction of said transistor when said amplifier output initiallyappears and said saturable core is saturating, said series diodeproviding protection against excessive voltage surges caused by thecollapse of the field surrounding said saturable core at the end of saidperiod in which said saturable core is saturating, said series diodethereby insuring that said period of nonresponse is always of the sametime duration.
 10. A fail-safe signal repeater for receiving andrepeating a signal from and to an electrically continuous rail, saidsignal repeater including, a. input means electrically coupled to saidelectrically continuous rail to receive said signal to be repeated, saidinput means having an amplifier, transformer coupled to said rail toreceive and amplify said signal from said rail, said amplifier having anoutput electrically coupled to a control means, b. said control meanselectrically coupled to said input means and a signal-repeating outputmeans, said control means having a triggered blocking oscillatorelectrically coupled to said amplifier output to be triggered thereby,said triggered blocking oscillator having an output signal electricallycoupled to said signal repeating output means, said output signal havinga length equal to a predetermined period of time, said blockingoscillator output signal providing a period of nonresponse for saidsignal repeater, c. said signal repeating output means having a storedsignal energy means as well as a switch circuit, said switch circuitelectrically coupled respectively to said blocking oscillator output andsaid stored signal energy means, said switch circuit responsive to theappearance of said blocking oscillator output signal to allow saidstored signal energy to be delivered to said electrically continuousrail as said repeated pulse.